Drag-and-drop From Program To Third-party Program?

I am trying to open multiple files using a third-party program, by calling it from VB.NET. By default, the program I am using, through command line, only opens one file at a time - and if I call it again, it opens a new instance. (when started in GUI, I can drop many files on it).

Now I open it with

Shell("program file")

If the program is running, I want to add "file" on top of the running instance instead of starting a new instance.

Something like:

If Not procExists then
Shell("program file")
else

[Code].....

Unfortunately, just like Shell("program filename"), it opened lots of instances of "program", instead o loading all the files in the same instance.